1. Field of the Invention
The present invention relates to a transistor circuit, and more particularly to a transistor circuit which comprises a plurality of transistor cells and is suitable for power amplification of a radio frequency signal.
2. Description of the Background Art
As is well known, a transistor circuit 101 for radio frequency power amplification is constructed such that a plurality of transistors 111 are connected in parallel with each other so as to keep a radio frequency characteristic, as shown in FIG. 6. In FIG. 6, a DC voltage (bias voltage) and a radio frequency signal are inputted to a base of each transistor 111. An emitter of each transistor 111 is grounded and an output signal of each transistor 111 is output from a collector to which the transistors 111 are commonly connected.
The transistor circuit 101 shown in FIG. 6 is an ideal circuit in the case of assuming that the respective transistors 111 uniformly operate, that is, there are no variations in operation. In practice, however, since there are characteristic variations and the like among the respective transistors 111, a positive feedback that “a temperature rise” causes “an operating current increase”, which in turn leads to “an increased temperature rise”, is made due to a positive correlation between an operating current and an element temperature which are specific to a transistor, thereby causing a phenomenon of currents concentrating in a certain transistor 111. The phenomenon causes problems that the transistor circuit 101 will have a gain or an efficiency reduced, and, at worst, some transistor 111 will have a high heating value during the operation thereby causing a thermal runaway, which may increase a base current of the transistor 111 thereby causing a device destruction.
In order to overcome such problems, a transistor circuit 102 is devised for preventing an increase in base current by providing a base ballast resistor 112 adjacent to the base of the transistor 111, as shown in FIG. 7. In the case of this circuit, however, although a thermal runaway of the transistor 111 is able to be prevented, the base ballast resistor 112 will reduce a radio frequency gain.
Therefore, in order to suppress the reduction in radio frequency gain, a transistor circuit 103 is suggested in which a capacitor 113 is connected in parallel with the base ballast resistor 112 as shown in FIG. 8. Such transistor circuit is disclosed in, for example, Japanese Laid-Open Patent Publication No. 8-279561 and the U.S. Pat. No. 5,321,279.
FIG. 9 shows examples of frequency—maximum power gain characteristic which are obtained by the transistor circuits 101 to 103 shown in FIGS. 6 to 8, respectively. FIG. 9 shows a simulation result obtained when the transistor 111 has an emitter area of 120 μm2 and an operating current of 2 mA, and the base ballast resistor 112 has a resistance of 200Ω, and the capacitor 113 has a capacitance of 0.3 pF.
When the configuration is as shown in FIG. 8, an impedance Z is represented by the following equation (1) where the base ballast resistor 112 has a resistance R and the capacitor 113 has a capacitance C:
                    Z        =                                            -              R                        ×                          j              (                              1                                  ω                  ⁢                                                                          ⁢                  C                                            )                                            R            -                          j              (                              1                                  ω                  ⁢                                                                          ⁢                  C                                            )                                                          (        1        )            
However, in a conventional configuration shown in FIG. 8 in which an impedance Z is represented by the aforementioned equation (1), it is necessary to set ωC(=2πfC) infinite so as to reduce the impedance Z to zero. Here, since a desired frequency f is fixed, a capacitance C may be made infinite in theory. However, it is impossible in practice. Specifically, a capacitance made on a semiconductor substrate cannot have a sufficiently great value due to a limitation on a chip layout.
Therefore, since a radio frequency power signal is inputted to the transistor 111 through not only the capacitor 113 but also the base ballast resistor 112, the base ballast resistor 112 causes a power loss, thereby reducing a radio frequency gain. Further, when a radio frequency signal passes through the capacitor 113, a harmonic component or an out-of-band signal component is not able to be reduced.